diff --git a/src/core.asm b/src/core.asm
index a19062f..a4e0a01 100644
--- a/src/core.asm
+++ b/src/core.asm
@@ -4,6 +4,11 @@ section .text
 	global	exit
 
 ;----- exit(exit_code) -----;
+; Exits the program with given exit code
+; Return value: N/A
+; Used registers:
+;	rax*	syscall
+;	rdi	(arg) code to exit the program with
 exit:
 	mov	rax, NR_exit
 	syscall
diff --git a/src/string.asm b/src/string.asm
index 2e069bf..4563754 100644
--- a/src/string.asm
+++ b/src/string.asm
@@ -1 +1,21 @@
 %include "src/constants.asm"
+
+section .text
+	global	strlen
+
+;----- strlen(*str[]) -----;
+; Gets the length of given string
+; Return value: Length of given string
+; Used registers:
+;	rax*	Byte to check in str[] >> (ret) length of given string
+;	rdi*	(arg) Pointer to str[]
+;	rcx*	Counter for scasb
+strlen:
+	xor	rax, rax
+	mov	rcx, -1
+	cld
+	repne	scasb
+	mov	rax, rcx
+	not	rax
+	dec	rax
+	ret
diff --git a/src/tests.asm b/src/tests.asm
index b9715c4..6870e54 100644
--- a/src/tests.asm
+++ b/src/tests.asm
@@ -4,8 +4,20 @@
 extern	exit
 ; console.asm
 ; string.asm
+extern	strlen
 ; convert.asm
 
+section .rodata
+	TEST_strlen		equ	1
+
+	; strlen()
+	strlenStr1		db	"Hello",EOS
+	strlenStr2		db	"Hello, world!",NL,EOS
+
+section .data
+
+section .bss
+
 section .text
 	global _start
 _start:
@@ -13,6 +25,19 @@ _start:
 	mov	rbp, rsp
 	sub	rsp, SIZE_QWORD
 
+;---
+;--- strlen()
+;---
+	; TEST 1
+	lea	rdi, [rel strlenStr1]
+	call	strlen
+	; TEST 2
+	lea	rdi, [rel strlenStr2]
+	call	strlen
+
+;---
+;--- exit()
+;---
 	leave
 	mov	rdi, 0
 	call	exit